Natural mechanical materials, such as bamboo and bone, often exhibit superior specific mechanical properties due to their hierarchical porous architectures. Using the principle of hierarchy as inspiration can facilitate the development of hierarchical mechanical metamaterials (HMMs) across multiple length scales via 3D printing. In this work, we propose self-similar HMMs that combine octet-truss (OCT) architecture as the first and second orders, with cubic architecture as the third or more orders. These HMMs were fabricated using stereolithography 3D printing, with the length sizes ranging from approximately 200 µm to the centimeter scale. The compressive stress–strain behaviors of HMMs exhibit a zigzag characteristic, and the toughness and energy absorption can be significantly enhanced by the hierarchical architecture. The compressive moduli are comparable to that of natural materials, and the strengths are superior to that of most polymer/metal foams, alumina hollow/carbon lattices, and other natural materials. Furthermore, the flexural stress–strain curves exhibit a nonlinear behavior, which can be attributed to the hierarchical architecture and local damage propagation. The relatively high mechanical properties can be attributed to the synergistic effect of the stretch-dominated OCT architecture and the bending-dominated cube architecture. Lastly, an ultralight HMM-integrated unmanned aerial vehicle (HMM-UAV) was successfully designed and printed. The HMM-UAV is ~85% lighter than its bulk counterpart, remarkably extending the flight duration time (~53%). This work not only provides an effective design strategy for HMMs but also further expands the application benchmark of HMMs.

Many processes may be used for manufacturing functionally graded materials. Among them, additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component. By adjusting the powder composition of the starting material layer by layer, a macroscopic and step-like gradient can be achieved. To further improve the step-like gradient, an enhancement of the in-situ mixing degree, which is limited according to the state of the art, is necessary. In this paper, a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting (LR) is described. The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process. Subsequent cross-sectional selective electron microscopic analyses were carried out. By applying LR, the mixing degree was enhanced, and the reaction zone thickness between the materials was increased. Moreover, an additional copper and iron-based phase was formed in the interface, resulting in a smoother gradient of the chemical composition than the case without LR. The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.

To fully utilize the in-situ resources on the moon to facilitate the establishment of a lunar habitat is significant to realize the long-term residence of mankind on the moon and the deep space exploration in the future. Thus, intensive research works have been conducted to develop types of 3D printing approach to adapt to the extreme environment and utilize the lunar regolith for in-situ construction. However, the in-situ 3D printing using raw lunar regolith consumes extremely high energy and time. In this work, we proposed a cost-effective melting extrusion system for lunar regolith-based composite printing, and engineering thermoplastic powders are employed as a bonding agent for lunar regolith composite. The high-performance nylon and lunar regolith are uniformly pre-mixed in powder form with different weight fractions. The high-pressure extrusion system is helpful to enhance the interface affinity of polymer binders with lunar regolith as well as maximize the loading ratio of in-situ resources of lunar regolith. Mechanical properties such as tensile strength, elastic modulus, and Poisson’s ratio of the printed specimens were evaluated systematically. Especially, the impact performance was emphasized to improve the resistance of the meteorite impact on the moon. The maximum tensile strength and impact toughness reach 36.2 MPa and 5.15 kJ/m², respectively. High-pressure melt extrusion for lunar regolith composite can increase the effective loading fraction up to 80 wt.% and relatively easily adapt to extreme conditions for in-situ manufacturing.

Selective laser melting (SLM) is a unique additive manufacturing (AM) category that can be used to manufacture mechanical parts. It has been widely used in aerospace and automotive using metal or alloy powder. The build orientation is crucial in AM because it affects the as-built part, including its part accuracy, surface roughness, support structure, and build time and cost. A mechanical part is usually composed of multiple surface features. The surface features carry the production and design knowledge, which can be utilized in SLM fabrication. This study proposes a method to determine the build orientation of multi-feature mechanical parts (MFMPs) in SLM. First, the surface features of an MFMP are recognized and grouped for formulating the particular optimization objectives. Second, the estimation models of involved optimization objectives are established, and a set of alternative build orientations (ABOs) is further obtained by many-objective optimization. Lastly, a multi-objective decision making method integrated by the technique for order of preference by similarity to the ideal solution and cosine similarity measure is presented to select an optimal build orientation from those ABOs. The weights of the feature groups and considered objectives are achieved by a fuzzy analytical hierarchy process. Two case studies are reported to validate the proposed method with numerical results, and the effectiveness comparison is presented. Physical manufacturing is conducted to prove the performance of the proposed method. The measured average sampling surface roughness of the most crucial feature of the bracket in the original orientation and the orientations obtained by the weighted sum model and the proposed method are 15.82, 10.84, and 10.62 μm, respectively. The numerical and physical validation results demonstrate that the proposed method is desirable to determine the build orientations of MFMPs with competitive results in SLM.

The internal structures of metallic products are important in realizing functional applications. Considering the manufacturing of inner structures, laser-based powder bed fusion (L-PBF) is an attractive approach because its layering principle enables the fabrication of parts with customized interior structures. However, the inferior surface quality of L-PBF components hinders its productization progress seriously. In this article, process, basic forms, and applications relevant to L-PBF internal structures are reviewed comprehensively. The causes of poor surface quality and differences in the microstructure and property of the surface features of L-PBF inner structures are presented to provide a perspective of their surface characteristics. Various polishing technologies for L-PBF components with inner structures are presented, whereas their strengths and weaknesses are summarized along with a discussion on the challenges and prospects for improving the interior surface quality of L-PBF parts.

High-entropy alloys (HEAs) are considered alternatives to traditional structural materials because of their superior mechanical, physical, and chemical properties. However, alloy composition combinations are too numerous to explore. Finding a rapid synthesis method to accelerate the development of HEA bulks is imperative. Existing in situ synthesis methods based on additive manufacturing are insufficient for efficiently controlling the uniformity and accuracy of components. In this work, laser powder bed fusion (L-PBF) is adopted for the in situ synthesis of equiatomic CoCrFeMnNi HEA from elemental powder mixtures. High composition accuracy is achieved in parallel with ensuring internal density. The L-PBF-based process parameters are optimized; and two different methods, namely, a multi-melting process and homogenization heat treatment, are adopted to address the problem of incompletely melted Cr particles in the single-melted samples. X-ray diffraction indicates that HEA microstructure can be obtained from elemental powders via L-PBF. In the triple-melted samples, a strong crystallographic texture can be observed through electron backscatter diffraction, with a maximum polar density of 9.92 and a high ultimate tensile strength (UTS) of (735.3 ± 14.1) MPa. The homogenization heat-treated samples appear more like coarse equiaxed grains, with a UTS of (650.8 ± 16.1) MPa and an elongation of (40.2% ± 1.3%). Cellular substructures are also observed in the triple-melted samples, but not in the homogenization heat-treated samples. The differences in mechanical properties primarily originate from the changes in strengthening mechanism. The even and flat fractographic morphologies of the homogenization heat-treated samples represent a more uniform internal microstructure that is different from the complex morphologies of the triple-melted samples. Relative to the multi-melted samples, the homogenization heat-treated samples exhibit better processability, with a smaller composition deviation, i.e., ≤ 0.32 at.%. The two methods presented in this study are expected to have considerable potential for developing HEAs with high composition accuracy and composition flexibility.

Aiming at the exploration and resource utilization activities on the Moon, in situ resource utilization and in situ manufacturing are proposed to minimize the dependence on the ground transportation supplies. In this paper, a laser-assisted additive manufacturing process is developed to fabricate lunar regolith composites with PA12/SiO₂ mixing powders. The process parameters and composite material compositions are optimized in an appropriate range through orthogonal experiments to establish the relationship of process–structure–property for lunar regolith composites. The optimal combination of composite material compositions and process parameters are mixing ratio of 50/50 in volume, laser power of 30 W, scanning speed of 3500 mm/s, and scanning hatch space of 0.2 mm. The maximum tensile strength of lunar regolith composites reaches 9.248 MPa, and the maximum depth of surface variation is 120.79 μm, which indicates poor powder fusion and sintering quality. Thereafter, the mechanical properties of laser-sintered lunar regolith composites are implemented to the topology optimization design of complex structures. The effectiveness and the feasibility of this laser-assisted process are potentially developed for future lightweight design and manufacturing of the solar panel installed on the lunar rover.